Data from: Songbird population trajectories diverge under simulations of conifer encroachment vs removal in a sagebrush ecosystem

Woody plant encroachment into grasslands and shrublands is a global
phenomenon, negatively impacting ecosystem services and wildlife
populations. North American sagebrush ecosystems have experienced
widespread degradation from encroaching conifers, leading to losses of
sagebrush-obligate wildlife. Removal of encroaching trees is a primary
restoration method, but whether management actions can influence wildlife
populations at management-relevant scales is rarely investigated. We
studied a local Brewer’s Sparrow in the Medicine Lodge Valley of southwest
Montana to understand how their territory occupancy and nest success were
impacted by tree and shrub cover. From 2019-2022, our data collection
resulted in 1,161 mapped territories and 449 nests, which we used to build
models of territory occupancy and reproductive productivity relative to
tree and shrub cover. We then used tree and shrub cover extracted from
historical imagery to estimate population size and productivity 70 years
in the past. Finally, using models of tree growth and expansion, we
estimated population size and reproductive productivity under two
simulated scenarios 30 years in the future: with and without restoration
through conifer removal. We observed that tree cover has more than tripled
at the study site since 1954, which our models predicted have caused an
almost 25 % decrease in the local population size and a 35 % decline in
offspring production. In a future scenario where tree removal is conducted
in areas with < 20 % tree canopy cover after 30 years, we predict
population size and offspring production will likely remain stable.
Alternatively, if tree cover is allowed to increase unabated, our
simulations predict a potential population decline of 60 %, with similar
losses to offspring production. We saw a stark divergence in the potential
futures of the local Brewer’s Sparrow population and implications for the
species at large. Continuing tree encroachment drastically decreases the
available habitat and causes a steep decline in population size. However,
proactive and continuing management of encroaching trees can alleviate
further losses in a species that has already experienced significant
range-wide declines. We also highlight the importance of encroaching trees
as a previously underappreciated conservation risk for sagebrush avifauna.

木本植物入侵草原与灌丛是一种全球性现象，会对生态系统服务与野生动物种群造成负面影响。北美艾灌丛生态系统（sagebrush ecosystem）已因入侵针叶树出现大范围退化，导致专性依赖艾灌丛的野生动物（sagebrush-obligate wildlife）种群数量下降。清除入侵木本植物是主要的生态修复手段，但目前极少有研究探讨管理措施能否在管理相关尺度上影响野生动物种群。我们在蒙大拿州西南部的梅迪辛洛奇谷开展了针对当地布氏鹀（Brewer’s Sparrow）的研究，以探明木本与灌丛覆盖度如何影响其领域占有率与繁殖成功率。2019年至2022年间，我们共收集到1161个已测绘的领域位点与449个鸟巢的数据，并基于这些数据构建了与木本、灌丛覆盖度相关的领域占有率与繁殖生产力模型。随后，我们利用从历史影像中提取的木本与灌丛覆盖度数据，估算了70年前该种群的种群规模与繁殖生产力。最后，我们借助树木生长与扩张模型，针对两种未来30年的模拟情景估算了种群规模与繁殖生产力：一种是开展针叶树清除修复的情景，另一种则未进行修复。我们观测到，自1954年以来研究区域的木本覆盖度增长了两倍以上，模型预测这已导致当地种群规模下降近25%，繁殖后代数量减少35%。在未来30年对树木冠层覆盖度低于20%的区域开展树木清除的情景下，我们预测种群规模与后代产量将大概率保持稳定。反之，如果任由木本覆盖度无限制增长，我们的模拟预测种群规模可能下降60%，后代产量也会出现同等幅度的减少。我们发现，当地布氏鹀种群的潜在未来走向，与该物种种群整体的保护启示之间存在显著分野。持续的木本植物入侵会大幅压缩可用栖息地，并导致种群规模急剧下降。不过，针对入侵木本植物开展积极且持续的管理，能够缓解这一已在其整个分布范围内出现种群大幅衰退的物种的进一步损失。我们同时强调，入侵木本植物作为一种此前未被充分重视的艾灌丛鸟类保护风险因素，其重要性不容忽视。